Aluminosilicate refractory fibers are commonly formed by melting a mixture comprising alumina and silica in a furnace and allowing the molten material to flow in a controlled stream from an orifice in the furnace to impinge upon the circumferential surface of one or more rapidly spinning rotors. The molten material initially adheres to the surface of the first rotor which it contacts. The rapid rotation of the rotor, however, causes the molten material to be flung from the surface of the rotor in small portions. Each portion so flung from the surface is either attenuated into a fiber or impinges on and initially adheres to the circumferential surface of another rotor in the rotor set, from which it is subsequently flung for attenuation into a fiber or to impinge again on the first rotor or on yet a third rotor. Normally air or steam is blown across the surface of the rotors in a direction generally parallel to the axis of the rotors. This air or steam flow serves to carry the attenuated fibers away from the vicinity of the rotors for collection into a fiber blanket or mat and to separate fiber "clumps." Any number of rotors may be used in a typical rotor set in order to obtain maximum fiber yield; in a number of installations either two or three rotors are used together. Typical rotor configurations are shown in U.S. Pat. Nos. 2,388,935; 2,428,810; 2,520,168; 2,520,169; 2,632,920; and 3,045,279. (In these patents the rotors are being shown for the purpose of fiberizing mineral wool. The same general type of rotor configurations are used for fiberization of refractory fiber, however.)
It has been known in the past to spin fibers from certain types of igneous rock melts using high rotational speed rotors; see U.S. Pat. No. 3,533,769. However, fiberization of aluminosilicate fiber has always posed different problems than fiberization of materials such as mineral wool fiber, rock wool fiber, glass fiber and the like, because of the nature of the compositions from which each is formed. Materials such as mineral wool fiber, rock wool fiber and glass fiber are formed from mixtures of a large number of inorganic oxides, including silica and alumina. Commonly among the oxides present in such materials are oxides which serve as fluxes. Presence of such fluxes in the fiber composition combined with the lower liquidus temperature ranges of these compositions allows the fibers to be attenuated more effectively and thus results in fibers of very small effective and average diameters.
The aluminosilicate fibers, however, are usually composed essentially entirely of alumina and silica or of alumina and silica plus non-flux oxides such as chromia, calcia and/or magnesia (see, e.g., U.S. Pat. Nos. 3,449,137 and 4,055,434). Such compositions are attenuated much less effectively than are the glass rock and mineral wool fibers, and therefore the fibers obtained are significantly coarser with greater average and effective diameters. Since the finer the individual fibers, the more efficient the resulting fiber blanket or other body is as a thermal insulation, it is desirable to reduce the average and effective fiber diameters of the aluminosilicate fibers to very small values. Various attempts have been made to accomplish this by different means of operation of the rotors, air or steam blowers, or other equipment used in the fiberizing process, but such attempts have resulted in only limited success. Consequently, some refractory fiber manufacturers have incorporated into their aluminosilicate compositions small amounts of fluxes such as soda and boria. While the use of such fluxes has a significant effect on the reduction of the average and effective fiber diameters, it has the disadvantage that the thermal properties of the resulting fibers, such as maximum service temperature, are significantly poorer than the properties of fibers made from mixtures containing essentially only silica and alumina and (if present) the non-flux oxides.
It would therefore be highly desirable to have a process for the production of aluminosilicate refractory fibers which would permit formation of fibers of both essentially aluminosilicate composition and very small fiber diameters. It would also be desirable for the process to result in a lower content of "shot" (unfiberized granules of feed material) in the fiber and also in a smaller particle size for the shot granules which are present.